Graphite coating films, coating films of metal salts or amides of fatty acids, waxes such as paraffin, silicone oil, and the like, or coating films containing ethyl cellulose, phenolic resin, or the like as a binder are conventionally formed on the surface of rubber layers of rubber-coated metal gaskets, or on the surface of rubber elastic sliding parts, such as bearing seals and oil seals, for the purpose of preventing fixing, inhibiting blocking, and improving abrasion resistance. Engine gaskets, and the like are used under the conditions of a high surface pressure and high temperature. When the vibration of an engine is further added to these conditions, the rubber coating layer on the surface of the gasket is abraded, and gas leakage may occur. Moreover, the rubber coating layer of rubber elastic sliding parts, such as bearing seals and oil seals, is abraded due to repeated sliding, and oil leakage may occur.
Accordingly, as a surface-treating agent for vulcanized rubber that hardly results in a phenomenon in which the rubber coating layer on the gasket surface is abraded or damaged even under severe service conditions, including high surface pressure and high temperature, as well as vibration, i.e., the use environment of engine head gaskets, and that can form a rubber layer of a gasket effective for gas sealing, the present applicant previously proposed a surface-treating agent for vulcanized rubber in which an aqueous dispersion of polyolefin resin is added to liquid hydroxyl group-containing 1,2-polybutadiene and isocyanate group-containing 1,2-polybutadiene as a curing agent thereof (Patent Document 1).
The surface-treating agent for vulcanized rubber proposed in Patent Document 1 achieved the intended purpose of improving the abrasion resistance of the rubber coating layer. However, due to the use of an aqueous dispersion, water promoted the reaction between the hydroxyl group and the isocyanate grouping the 1,2-polybutadiene, and water also reacted with the isocyanate group itself; thus the viscosity of the surface-treating dispersion was increased, and gelation occurred. This caused the problem of poor workability during the coating process. Furthermore, as a result of the reaction between water and the isocyanate group, importing high molecular weight to 1,2-polybutadiene was prevented, and there were drawbacks that the abrasion resistance, peeling resistance, and sliding properties of the surface treatment layer were inferior.
In order to solve the above problems, the present applicant further proposed a surface-treating agent for vulcanized rubber that uses an organic solvent dispersion of polyolefin resin in place of an aqueous dispersion of polyolefin resin (Patent Document 2). However, this method did not use a fluororesin, such as polytetrafluoroethylene, and there was thus a problem that the coating film was peeled due to adhesion during high temperature compression, and friction and abrasion at high surface pressure.
As a surface-treating agent for vulcanized rubber that does not lead to the peeling of a coating film due to adhesion during high temperature compression, and friction and abrasion at high surface pressure, without impairing the performance conventionally required for surface-treating agents for vulcanized rubber, such as prevention of fixing, inhibition of blocking, and improvement of abrasion resistance, the present applicant further proposed a surface-treating agent for vulcanized rubber comprising an organic solvent solution comprising 10 to 160 parts by weight of wax having a softening point of 40 to 160° C. and 10 to 160 parts by weight of fluororesin, based on 100 parts by weight of isocyanate group-containing 1,2-polybutadiene or a 1,2-polybutadiene mixture of isocyanate group-containing 1,2-polybutadiene and hydroxyl group-containing 1,2-polybutadiene (Patent Document 3).